Hi Robin.
Definitely do not commingle funds, and by that I mean do not have all your personal and LLC funds going into and out of the same bank account. There is a legal concept called "piercing the corporate veil" (applies to LLCs also), and commingling funds is one of the surest ways to lose your liability protection.
You should have cash reserves when you create the LLC (again because the "piercing the corporate veil" concept), but that does not mean enough money to cover all unforeseeables. I tend to think 2 months of expected operating expenses is enough. You can always put in more money.
If you have $10k in reserves and due to unexpected repairs you need to put in $5k more, you can do so in 1 of 2 ways. You can either make a loan to the LLC as you suggested, in which case there should be a note, regular payments, and interest at least at AFR. Your other option is to make a capital contribution to the LLC. You would simply have the LLC book it as a contribution, eventually record it that way on your tax return, and you can always take the money back out as a distribution later.